In recent years, portable communication terminals, such as mobile telephones or PDAS (Personal Digital Assistants), have begun to make various information transmission services available to people. The information transmission services include audio communication service, SMS (Short Message Service, electronic mail service, and information delivery service using a wireless Internet. In the present mobile communication system, however, if a large amount of data, such as moving picture data, were transmitted, transmission channel band would become a bottleneck, making it impossible to transmit the data as it is.
Thus, to transmit moving picture data by radio, a moving picture compression/expansion process is needed. As one of measures for realizing the process, use of an image compression/expansion scheme called MPEG-4 is under examination.
In MPEG-4, data correlating with each other on the time base, such as moving picture data, are represented by the initial frame and difference frames. The initial frame is referred to as an I frame (Intra-coded Frame) and includes all the image information elements about one screen obtained by compressing a picture into a still picture. That is, an I frame constructs an image frame that makes detect independently by itself.
In contrast, a difference frame is referred to as a P frame (Predictive-coded Frame) and includes mainly information on the difference between the image of the present frame and that of the preceding frame. With the first frame timing after the start of transmission, an I frame is transmitted. With each subsequent frame timing, P frames are transmitted sequentially. Use of MPEG-4 enables moving picture data with the amount of information equivalent to, for example, several Mbps, to be compressed to several tens of Kbps for transmission.
On the other hand, in the mobile communication system, the reception field strength at a mobile station varies greatly, depending on the positional relationship with the base station and its environmental conditions. FIG. 10 shows an example of the great fluctuation. When the reception field strength has dropped below the level represented by a broken line H necessary to secure a specific error rate, the mobile station cannot reproduce the received data properly.
For example, when moving picture data is transmitted using MPEG-4, if a period in which the reception field strength is below the level is relatively short as shown by T1 in the figure, noise appears in the reproduced image only temporarily. If the reception field strength is below the level for a long time as shown by T2 in the figure, however, errors occurred in a plurality of P frames are accumulated one after another. As a result, the quality of the reproduced image deteriorates seriously. Once many errors have been accumulated over a plurality of frames, even if the reception field strength has been restored and a correct P frame is received, a good image cannot be reproduced immediately, because the P frame is information on the difference between the present frame and the preceding frame. Therefore, a lot of time elapses until a good image can be reproduced.